System of compensating automatic volume control



Jan. 4, 1938. R. HOLLINGSWORTH 2,104,324

SYSTEM OF COMFENSATING AUTOMATIC VOLUME CONTROL' 4 wmv-Huw @d K lNvENToR v RLEE HOLLINGSWORTH 7 v AM ATTORNEY.

Jan. 4, 1938. R. HOLLINGSWORTH 2,104,324

SYSTEM OF COMPENSATING AUTOMATIC VOLUME CONTROL Filed June 27, 1934 44 Sheets-snaai 2 :n 1E :1 -IvwwvL-n Til" Q il -f-Q k l R um ,s

- Q: T `l Y Q s@ l v @i @E INVENTOR w n MEE HOLLINGSWORTH w -f\ Tay/gm I 'Hu" I ATTORNEY y Jani 4, 1938. R. HOLLINGSWORTH l 2,104,324

SYSTEM oF COMPENSATING AUTOMATIC VOLUME CONTROL- lFiled June 27, 1934 4 Sheets-Sheet 3 INVENTOR R LEE HOLLINGSWDRTH Il BY i ATTORNEY Jan. 4, 1938. A R. l.. HOLLINGSWORTH 2,104,324

SYSTEM OF COMPENSATING AUTOMATIC VOLUME CONTROL- Filed June 27, 1934 4 sheetsfsheet 4 1' t 'Q f w Q www :ma VW Q d NQS E Q N no 52704 i INVENTOR y Rue HoLuNeswoRrH #www ATTORNEY Patented Jan. 4, 1938 UNITED STATES SYSTEM OF COMIVENSATING AUTOMATIC VOLUME CONTROL R. Lee Hollingsworth, Riverhead, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application June 27, 1934, Serial No. 732,565

9 Claims.

This invention relates to signalling devices of the type in which signal modulations on a carrier wave are converted into keying impulses` and in which automatic volume control means is provided for maintaining the level of the signal comparatively constant irrespective of fluctuations in signal energy received and in which additional means is provided for controlling the automatic Volume control means, so that a con- ]0 tinuous marking impulse will not be produced by the system in the absence of signal carrying waves in the receiver.

When automatic volume control means is used with high gain receivers the full gain or amplification of the receiver may be applied suddenly upon the output circuit which may be a loud speaker, visual indicator, a line, or a telegraph keyer or any other apparatus, when the carrier energy is reduced to an extremely low value or when the carrier is completely removed.

This operation of the automatic Volume control on receivers of radio telegraphy waves produces undesired results because if the automatic volume control is sufficiently fast, that is, of a small enough time constant to allow the receivers gain to follow deep and fast fading properly, as the full gain of the receiver is attained due to a falling off of the energy in the received wave a large amount of energy is impressed upon the keyer. The same result occurs when the transmitting station ceases to send out signals for a short period of time. When the full gain, including the existing noise amplified to a Very high level, is impressed or applied to the keyer, it causes a steady direct current impulse or tone, or in other words a continuous marking impulse, to now through the line to the recording apparatus. This is a very undesirable condition and among other things leads the operators to believe that insofar as they can judge the transmitting station is sending out a continuous dash, indicating a possible source of trouble within the transmitting circuit.

An object of the present invention is to provide a practical method of and means for overcoming the productionof this continuous marking impulse when the received signal strength falls below a useable value or the transmitter ceases sending. Brieiiy, this object of the present invention is accomplished by the use of a biasing potential supply in addition to the potential supply provided for automatic volume control purposes. This additional supply increases and decreases in amplitude as the supply of potential heretofore utilized for automatic (Cl. Z50-8) volume control purposes decreases and increases. Each of the biasing potential sources acts to supplement the action of the other source and consequently the amount and sensitivity of the automatic volume control.

A feature of the present invention is that in addition to controlling the receiver so that the absence of signals or the falling olf of the amplitude of the signals does not produce what appears to be a continuous dash in the output of the converter Ybut does increase the sensitivity of the keying device and insures a more complete signal to be obtained from a varying Wave. Moreover, the present method and means permits the sensitivity and the gain of the converting device to be regulated over a range sufficient to operate successively on signals of varying strength and character. The novel features of my invention have been pointed out with particularity in the claims appended to the specification.

The nature of my invention and the manner in which the same may be utilized will be better understood from the following detailed description thereof and therefrom when read in connection with the attached drawings throughout which insofar as possible like characters indicate like parts and in which; Y Y Figure 1 illustrates a preferred embodiment of a circuit arrangement including the essential elements of a receiver including my automatic volume control and compensating means; Y

Figures 2 and 3 illustrate modifications of the arrangement of Figure 1, while;

Figure 4 is a graph illustrating the operation of the devices illustrated in Figures 1 to 3 inelusive.

Referring to the drawings, and in particular to Figure 1, A, is an aerial system connected with a radio frequency amplifier, demodulator and audior frequency amplifier B. In practice the aerial system A may be replaced by a transmission line of the desired character. The radio frequency amplifier in the receiver B may be of the tuned radio frequency type or of the heterodyne type or any other known type. Energy from the output of the demodulator in B is applied directly or by Way of audio frequency amplifiers in B to the input of a rectifier C. The rectifier C may be of any type known. Rectified current from the output of the rectifier C is passed through a low pass lter LP comprising series inductances and parallel capacities connected as shown. The filtered energy from the output of the filter LP is passed through a resistance Ri. A keying tube T2 has its control grid connected as shown to a point on the resistance Rl. The anode of T2 is connected as shown by way of a source of potential and a resistance R5 to ground and from ground to the -cathode or" T2. A point on the anode circuit of T2 is connected to the electrical center of the secondary winding of a transformer T4, the primary windings of which are connected as shown to a source of tone frequencies TG. The terminals of the secondary winding of T4 is connected to the control grids of a, pair of thermionic tubes l0 and i2. The output electrodes of these tubes are coupled by a transformer T5 to any utilization circuit such as a long line or a recording device. Received signals are rectified in C, The rectified signal impulse components are smoothed in LP and appear across Rl. The control grid of T2 is biased to a point at which the tube is normally conductive so that current flows in the anode circuit and consequently in the resistance R5. The flow of signal impulses through Rl produce a potential drop so that the control grid of T2 becomes less positive thereby cutting the flow of current in the anode circuit and consequently in the resistance R5. This causes a rise of positive potential on the control grids of tubes ID and i2 with respect to their cathode potential, which are connected to a point on the anode circuit of tube T2. This rise of potential causes the tubes l@ and i2, which are normally biased, to cut off to become conductive and permit oscillations at tone frequency to be transferred from the source TG to the outgoing line for keying or recording purposes.

The output of the low-pass lter LP is also connected by way of a resistance R2 and a line AVC to the unit B, and/or to the unit C, to supply thereto a direct current potential deterrm'ned by the intensity of the received wave for control purposes. This direct current potential may be applied to the control grid of one of the radio frequency amplifiers or any other tube in the unit B, or to the grid of a rectifier in C, (grid controlled rectification) to render the same less negative when the amplitude of the wave therein falls off and more negative should the amplitude of the Wave therein exceed a predetermined value. A condenser TC which in part determines the time constant of the circuit is connected as shown between a terminal of R2 and ground.

in order to prevent the falling ona of signal wave intensity or the stopping thereof, from producing the effect of a continuous marking impulse in the receiver I provide a compensating circuit which will now be described. The compensating circuit comprises a control tube Tl having its control grid connected as shown between the rectifier C and the low-pass lter LP. The anode of tube TI is connected as shown by way of a source of potential and a choking inductance and a resistance R4 to the cathode of TI. Tl, like T2, is normally biased to a potential such that anode current flows through the anode circuit including the resistance R4. Whn signals appear in C, however, current iiows through the circuit to produce a potential drop therein which renders the grid of Tl less positive, so that less or no current flows in the anode circuit of Tl. The potential drop across R4 is supplied by way of a resistance R3 to the condenser TC to determine in part the charge on said condenser. That portion of the resistance R4 between the point on the potentiometer and ground is connected as shown in shunt with TC and is in eifect in parallel with the circuits R2, Ri which is also in parallel or in shunt with TC. When there is no signal voltage drop across Rl, R2, the voltage drop across R4 charges the condenser TC and produces a cw ci current through R2, Rl. Tl and T2 may draw some grid current which also flows through Rl. This current produces a drop through Rl which is added to the drop produced therein by the current now, due to the potential in R4. Grid current in TI, T2 is not necessary for satisfactory operation. The drop of potential caused in Ri by anode current from Ti supplied from R4 through R3 and R2 is small due to the proportionate values oi these resistances.

As indicated above, when no signal is present, the flow of current, and consequently the fall of potential through Rl, is determined by the drop through R4 alone or by the same supplemented by the potential produced in Ri by grid current in Ti, T2. flows through Rl and the grid of Tl becomes less positive so that grid current ceases to flow and the anode current is reduced to zero, or to a desired level. The potential drop through R4 is small so that no potential is applied to the condenser TC, for charging purposes. The condenser TC then discharges through R3 and R4 and to some extent through R2 and R! the parallel path to ground until the iiow of current in Rl due to the incoming signal reaches a value Il* such as will reverse the current ow from RI through R2 and start to recharge condenser TC with signal voltage. This circuit is completed from R2 by Way of R3 and R4 to ground. Thus when the signal Voltage is 10W there is a parallel discharge path from the condenser TC to effectively increase the rate of the receivers gain when it is needed.

The main object or" connecting the control grid of the Control tube TI is to insure that Tl is controlled; that is, biased or actuated suliiciently ahead of T2 to allow the gain of the receiver as determined by the automatic volume control to start increasing by the time the current passes through the filter to cperate the keying tube T2, in case the mean signal strength at this moment is very weak. This action canronly be had when R2 and R3 are comparatively small in value, causing the time lag circuits TC, Rl, R2, R3, Rt to be rather fast so that the change in the gain of the receiver is almost instantaneous.

The operation of the receiver will now be eX- plained. In explaining the operation three conditions of operation will be utilized.

First it will be assumed that no signal is being received. The movable point on Ri is adjusted to a point somewhere near the above ground end of Rl and the movable point on R4 is adjusted to a point such as to cause the of the receiver as determined by the automatic volume control circuits to be reduced to a point where noise peaks occasionally energize T2 sufficiently to render the coupling tubes i@ and I2 conductive. Now if these points have been properly selected when the current ows through Rl it will produce across Ri the exact amount of potential drop as is built up across Ri and R2, due to the potential applied thereto from R4 when there is incoming signal current. Under these circumstances the gain of the receiver remains the same, except that the Ygain rises slightly at the start of all signal impulses, be they stronger or weaker than that required to produce through Ri a voltage drop lesser than equal to, or greater than, that pro- The instant a signal is received current ahead of the filter L? i ceiver.

Y 2,104,324 duced therein by the potential drop in R4. This particular action is shown graphically in Figure 4. When this condition exists, the automatic volume control supply simply changes from RI to Ril without changing the gain of the receiver eX- cept as explained at the start of the change of the bias supply from Rt to Ri.

Under the second conditions of operation it will be assumed that I am using the same adjustments as used above. Now assume that the signal energy produces a drop through Ri which. is greater than the drop supplied by R4.V The charge on the condenser TC is lessened at the start of the signal impulse, and the gain of the receiver increases for an instant as the signal voltage starts building up across RI As the signal voltage increases the change of gain in the receiver is halted and nally reversed as the potential drop in Rl begins to approach the drop supplied therein by R4 and the gain is further reduced during this impulse in proportion to the strength of the signal. Between each signal impulse the gain rises, or starts to rise, to the value originally selected by adjustment of the movable point on R4. The speed of keying and the time constant of the automaticY volume control circuits both govern the amount of gain that the receiver is allowed to increase between impulses.

Under the third condition of operation it will be assumed that the circuits have the same adjustments as before. However, it will be assumed that the potential Vdrop across Rl is somewhat less than the potential drop supplied thereto from 4. Under these circumstances one must keep in mind that the line keyer tube T2 operates on a signal strength of one microvolt input to the re- The signal strength under such circumstances is almost always below the noise level. The automatic volume control tube Tl is cut off first by the flow of potential in RI due to received signal and because of reduced plate voltage, and further because this tube is energized rst. VTherefore, when the signal voltage drop across Rl is less than `that supplied from R4 the condenser TC discharges more rapidly through RS and R4 and through R2 and Rl, the percentage of discharge through R2 and Rl depending upon the actual potential build up across Ri by the received signal voltage. Here the gain of the receiver is increased during each signal impulse and decreased during the interval of time between impulses.

The overall receiver performance, as explained in connection with conditions l, 2 and 3 are extremely desirable. The advantages obtained .from this novel keying system are numerous. Under conditions l and 2, during initial adjustment the device is so set that the keyer operates as close to the noise level as practical. Due to the increased gain, as explained in condition 3, the signal even when fading below this value does not produce dropouts in the line signal because the gain of the receiver is increased with each impulse and consequently the signal can work into the noise level without the line signal actually stopping except under extreme conditions of fading. A large amount of the noise passed through the keyer on deep fades is ltered out in the line circuits following the coupling tubes lil and l2 so that in practice a practically clean and complete signal is obtained even though the incoming signal strength falls below the noise level.

The arrangement shown in Figure 2 is in many respectsl similar to the arrangement shown in Figure 1. However, in the arrangement of Figure 2 the control grid of TI is connected to the same po-int on the resistance RI that the control grid of T2 is connected. The operation of TI ahead of T2 is insured here due to the reduced voltage applied to the plate of T2. The tube T2 drawsv its plate voltage from a voltage divider VD which is connected as shown across a common high potential source to supply the entire apparatus except for the cathode heating sources and the potential source PS connected with the anodes or the tube Tl. The arrangement of Figure 2 is simplified in other respects asshown. The screen grids of tubes TI and T2 may be con,- nected as shown to a second Voltage divider VD2.

In both circuits Ti and T2 draw grid current. In Figure l the grid current may be utilized as a fine adjustment of R4, because raising the arm of point on Rl more above ground adds negative voltage to the charge applied to TC, thereby reducing the receivers gain and decreasing the grid current of the tone keyer T2 and likewise making it more sensitive simultaneously as the receiver gain is decreased. Where both grids are commonly connected to the arm of Rl as in Figure 2 the amount of current drawn by the gridv circuits of the tubes may be decreased as the point on RI is moved further above ground.

In this manner the plate current of TI and T2 may be reduced to a point at which they are cut ofi readily. In this manner the sensitivity of the keying tube may be adjusted to as great a value as may be desired, but does not affect materially the gain of the receiver.

In many cases the necessity of using an additional tube T2 for control purposes is not necessary. For example I may utilize an arrangement as shown in Figure 3. In the arrangement of Figure 3 the single tube T2 serves as a keying tube to supply biasing potentials to the coupling tubes lil and I 2 to control the tone impulses sent out from the source TG over the line. The resistance R4 in` this circuit supplies the supplemental potentials for charging the condenser TC and for controlling the potential drop 'Ihe circuit otherwise is somewhat similar to the arrangement of Figure l. This circuit, however, does not possess the feature of having the space bias operating in advance of he line keyer tube.

In operation when no signal is received the cony trol grid of T2 is suicie-ntly positive to permit a iiow of current in the anode of T2 and consequently in the resistance R4. The drop of potential through this resistance biases the tubes iii and l2 to cut off so that no tone impulses can go out over the line. At the same time the potential drop through Ri is applied through resistance R3 to the automatic volume control circuit to supplement the action of 'Il as in Figure l. The time constants of the automatic volume control circuit should be such as to permit the automatic volume control to act rapidly to increase the gain of the receiver as the keyer tube starts to cut ofi. On the reception of signals current ilows in RI. The potential drop produced by said current in Rl is applied to the control grid of T2 and the flow of current in the anode circuit thereof decreases. The potential drop through Ri decreases and the control grids of tubes it and l2 become more positive thereby permitting tone frequency impulses to be impressed on the line from the source TG. At the same time, there is less potential drop applied by coY way of resistance R3 across TC and this condenser 75 is permitted to discharge through RI and R2 depending upon the signal strength and the gain of the receiver varies inversely with the strength of the received signal.

The overall characteristics of my novel tone keyers of Figures l and 2 are shown in Figure 4 of the drawings. In this figure the current through R! (abscissas) is plotted against the plate current (ordinates) of the keyer tube T2 and of the AVC tube Ti and also against line volts at the output of the system.

Having thus described my invention and the operation thereof what I claim is:

1. The combination with a source of signal carrying waves, of means for converting said waves into direct current impulses characteristic of the signals comprising, a rectifier connected to said source, a low-pass lter connected to said rectifier, a discharge tube having its input electrodes connected to the output of said low-pass filter, a utilization circuit connected to the output of said tube, an automatic volume control circuit having two parallel branches, one of which includes said low-pass filter and a resistance, and the other of which includes an additional tube and a resistance, and a connection between said automatic volume control circuit and said source of signal carrying waves.

2. The combination with signal carrying Wave relaying means of the discharge tube type, a rectifier connected with said relaying means, an output circuit connected with said rectifier, a gain control device comprising a resistance and a condenser connected with the output of said rectifier and to a control electrode in a tube of said wave relaying means to control the relaying characteristic thereof in accordance with the amplitude of the wave, and a device for preventing said gain control from causing said relay to energize said rectifier and said output circuit continuously in the absence of wave energy in said relaying means comprising a circuit including potential reversing means connecting in parallel with said resistance and condenser.

3. 'Ihe combination with signal carrying Wave amplifying means of a rectifier connected with said amplifying means, a filter circuit connected with said rectifier, an output tube, a control tube connecting said rectifier to a control electrode of said output tube to determine the conductivity thereof, a gain control device comprising, a resistance and a condenser connected with the output of said filter and to control means in said wave amplifying means and a device for preventing said gain control from maintaining said output tube conductive continuously in the absence of Wave energy in said amplifying means comprising a circuit including a potential reversing means connected with the output of said rectifier and to said resistance and condenser.

4. The combination with signal carrying wave amplifying means of a rectifier connected with said amplifying means, an output tube, a control tube connecting said rectifier to a control electrode of said output tube to determine the conductivity thereof, a gain control device comprising, a resistance and a condenser connected with the output of said rectifier and to said wave amplifying means to increase the gain thereof when the wave amplitude decreases and vice versa, and a device for preventing said gain control device from maintaining said output tube conductive continuously in the absence of Wave energy in said amplifier means comprising a thermionic tube having its input electrodes connected to said rectifier and its output electrodes connected by Way of an impedance to said resistance and condenser.

5. In a signal keyed wave recording means, a wave amplifier, a rectifier coupled to the output of said amplier, an impedance connected to the output of said rectifier, recording means controlled by the potential drop in said impedance, an automatic volume control circuit including an impedance and a condenser connected with the output of said rectifier and to said amplifier to control the gain thereof, and means for preventing said automatic volume control circuit from unduly increasing the gain of said amplifier in the absense of signal markings on said keyed wave in said amplifier, comprising an additional impedance connected to the output of said rectifier, and means connecting said last named impedance in shunt to said condenser to control the charge of said condenser in a sense opposite that to which the charge of said condenser is controlled by signal currents in the output of said rectifier.

6. A signal wave recording means as recited in claim 5 in which a filter circuit is connected between tlie output of said rectifier and said first named impedance.

7. In a signal wave recording means, a Wave amplier of controllable gain, a rectifier connected with the output of said amplifier, a filter circuit connected to the output of said rectifier, an impedance connected across the output of said lter circuit, a recording circuit connected to said impedance, an additional impedance connected to the output of said filter circuit, a condenser connected in shunt with said last named irnpedance, a circuit connecting said last named impedance to said amplifier to apply thereto a potential to control the gain thereof, a thermionic tube having its control grid connected to the output of said rectifier, an impedance connected between the output electrodes of said thermionic tube, and a circuit connecting a point on said last named impedance to said second named impedance.

8. In signal wave recording means, wave amplifying means of controllable gain, a rectifier connected with the output thereof, a filter circuit connected to the output of said rectifier, a pair of impedances connected with the output of said filter circuit, a thermionic tub-e having its control grid connected to a point on one of said impedances, an additional impedance connected between the output electiodes of said thermionic tube, a recording circuit connected to a point on said additional impedance, a connection between a point on said additional impedance and a point on the other of said first named impedances, and a connection between the other of said first named impedances and said amplifier for applying a potential thereto to control the gain thereof.

9. In combination, an antenna, a radio frequency amplifier coupled to said antenna, a detector for detecting the output of said amplifier, an automatic volume control circuit responsive to the output of said detector for controlling the amplification of said radio frequency amplifier, a vacuum tube, means coupling input electrodes of Vsaid vacuum tube to said detector, a resistance connected between the output electrodes of said vacuum tube, and means variably connected to said resistor for decreasing the amplification of said radio frequency amplifier in the absence of signal upon said antenna.

R. LEE HOLLINGSWORTH. 

